The U.N. says ISIS has abducted tens of thousands of civilians whom it will likely use as human shields in the city of Mosul.

The extremist group has forced almost 8,000 families to leave their homes, probably as a tactic to defend Mosul from the U.S.-backed offensive launched last week, a spokesperson for the U.N. High Commissioner for Human Rights said on Friday.

ISIS fighters were also allegedly killing civilians who refused to comply with instructions or had belonged to the country's security forces. Some 232 civilians were reportedly shot to death last week alone, Ravina Shamdasani, the spokesperson, told a briefing.

"The use of human shields is prohibited under international humanitarian law and constitutes a violation of the right not to be arbitrarily deprived of life," she said.

ISIS' forced relocation of families has doubled the population of its stronghold of Hamam al-Alil, a town around 10 miles south of Mosul, which now houses an estimated 60,000 people, she added.

ISIS is trying to use the civilians' presence to deter the Iraqi-led, U.S.-backed coalition from attacking its positions, knowing that doing so may result in additional innocent casualties.

"Fighters are allegedly killing civilians who refuse to comply," according to Shamdasani. "Forced out by gunpoint, or killed if they resist, these people are reportedly being moved to strategic locations."

Shamdasani also revealed that last Wednesday, days after the campaign to recapture Mosul kicked off, ISIS fighters shot dead 232 civilians at two military bases near the city.

Most were former members of the Iraqi security forces who were shot dead at the al-Ghazlani military base, the others were civilians shot in the head at another base after refusing to join the extremists, Shamdasani said.